The present invention relates to a technology for correcting a frequency offset in a reception-system circuit of a wireless communication system, and to, for example, a technology effective for application to a frequency offset cancel circuit provided in a reception-system circuit of a wireless communication system which adopts a frequency hopping method.
In transmission of data by a wireless communication of an FM-modulation scheme, a carrier wave (carrier frequency signal) is frequency-modulated by transmit data to thereby superimpose signal components on the carrier wave, whereby the transmission of the data is carried out. A transmitting side performs the generation of the carrier wave and superimposition of the signal components on the carrier wave and outputs the resultant data, whereas a receiving side eliminates a carrier component from a received signal to thereby extract a signal component. Further, the receiving side demodulates data of “0” and “1” (correspondence between high and low levels of frequency and “0” and “1” is arbitrary) according to whether the frequency of the received signal is higher or lower than the frequency of the carrier wave. The generation and elimination of the carrier wave are generally carried out using oscillation signals of VCOs (Voltage-Controlled Oscillators) respectively provided on the transmitting side and the receiving side.
However, the VCOs on the transmitting and receiving sides have variations respectively. A frequency offset is developed in the received signal. Further, there may be cases in which an offset lying in a given range is allowed for each of oscillation frequencies of the respective VCOs on the transmitting and receiving sides depending on specs of wireless communications. In a Bluetooth standard that sets rules on the transmission of data by wireless communications among peripheral devices such as a personal computer, a printer, etc., an offset of 75 kHz is allowed for the transmission system as an initial offset, an offset of 50 kHz is allowed for the reception system, and an offset of 125 kHz in total is allowed for the receiving side.
If the offset component is not corrected on the receiving side in such a case, then a frequency value used as a decision threshold value of “0” or “1” is not determined and hence the receive or received data cannot be demodulated properly. An example illustrative of a signal waveform containing an offset is shown in FIG. 15. In FIG. 15, a vertical axis indicates a frequency modulating direction, and a horizontal axis indicates a time base. When no offset is contained in the signal waveform, “0” and “1” of data can be determined according to whether a modulation level is higher or lower than a frequency modulation level with the frequency modulation level “0” as the reference. In the signal waveform shown in FIG. 15, however, the modulation level of the whole received signal is shifted in a + direction due to the influence of an offset component. Thus, the determination may preferably be made to such a waveform as shown in FIG. 15 by setting a determination level of “0” or “1” (threshold) to a level of 3.0.